Rather than having equivalent DEF, I think it'd be more interesting if wizard equipment sacrificed the defensive enhancements endemic to armour but in their place added considerable bonuses to spellcasting. Simple things like additional INT (mind you, INT increases spell resistance too!) and spell damage modifiers (Perhaps different robes bolster different elements) all the way up to additional mana generation and special abilities. (wands that shoot a weak Flame Dart-esque attack, for instance, but require a certain INT to use)
Of course, this is all part of my desire for far more interesting magical items in the first place. But I think the most important thing of this, and of all changes made for the game, is to give the player choices. Important and interesting ones.
